As is well known, an optical signal may have two orthogonal polarization states, each of which may have different properties. Sometimes such polarization states are intentionally introduced, such as in creating a polarization-multiplexed signal in which the two orthogonal polarization states of the optical carrier are arranged so that each carries different data in order to double the spectral efficiency. Such a polarization-multiplexed signal has two so-called “generic” polarization components, each of which carries a single data modulation. Note that by a generic polarization component it is generally intended the signal at the point at which the modulation of that polarization component is completed. It should be appreciated that each generic polarization component may initially, or otherwise, exist separate from the other generic polarization component with which it is later combined.
The polarization orientations of the generic signal components are generally changed by the birefringence of the fiber, and possibly other fiber properties, during the passage of the signal over the optical path. Such changes may be time varying because at least the fiber birefringence is typically a function of various factors such as ambient temperature, mechanical stress, and so forth, which may vary over time and be different at various points of the transmission path. As a result, the polarization orientation of each of the generic signal components is generally unknown at the receiver.
Sometimes, undesirably, the fiber birefringence is so large that polarization-mode dispersion (PMD) is caused, i.e., a generic optical signal component is decomposed into two orthogonal polarization components along the two principle state of polarization (PSP) axes of the fiber, along one of which the light travels at its fastest speed through the fiber and along the other of which the light travels at its slowest speed through the fiber. In such a case, not only may the phase relationship between the two polarization components be time varying, but also each of the two orthogonal polarization components may arrive at the receiver at different times due to the PMD-induced differential group delay (DGD) between the two PSP axes. Note that, actually, as suggested above, each small section of the fiber behaves as if it is its own mini fiber that introduces its own DGD between the two PSP axes. However, for simplification purposes, one may treat the fiber as a single DGD element that introduces a certain DGD between the two axes, based on a first order approximation of the PMD. Thus, for a particular fiber or optical link, PMD is a stochastic effect, and the PMD-induced DGD may also be time varying.
Other linear effects distort optical signals transmitted over optical fibers. Such effects include chromatic dispersion (CD). Optical compensation methods are typically employed to reduce signal distortion that arises due to CD or PMD.
Electronic chromatic dispersion compensation (EDC) has recently emerged as a technique that can flexibly reduce the distortion induced by CD in a cost effective manner. As explained by M. S. O'Sullivan, K. Roberts, and C. Bontu, in “Electronic dispersion compensation techniques for optical communication systems,” ECOC'05, paper Tu3.2.1, 2005, EDC can be performed at the transmitter. Alternatively, EDC can be performed at the receiver. As described by S. Tsukamoto, K. Katoh, and K. Kikuchi, in “Unrepeated Transmission of 20-Gb/s Optical Quadrature Phase-Shift-Keying Signal Over 200-km Standard Single-Mode Fiber Based on Digital Processing of Homodyne-Detected Signal for Group-Velocity Dispersion Compensation,” IEEE Photonics Technology Letters, Volume 18, Issue 9, 1 May 2006, pp. 1016-1018, EDC is implemented with a coherent-detection receiver. In addition, EDC can be implemented with a special direct differential detection receiver as explained by X. Liu and X. Wei, in U.S. patent application Ser. No. 11/525,786 entitled “Reconstruction and Restoration Of Optical Signal Field”, filed on Sep. 22, 2006 and assigned to Lucent Technologies, which is incorporated by reference as if set forth fully herein and shall be referred to hereinafter as Liu-Wei.
Unlike CD, PMD in a fiber link may change very rapidly and PMD compensation usually has to be done in the receiver. Electronic PMD compensation (EPMDC) has also attracted attention recently for its potential cost effectiveness. As explained by J. Hong, R. Saunders, and S. Colaco, in “SiGe equalizer IC for PMD Mitigation and Signal Optimization of 40 Gbits/s Transmission”, published in Optical Fiber Communication Conference 2005, paper OWO2. However, the capability of the EPMDC with a conventional direct-detection receiver is quite limited in that the improvement in PMD tolerance is usually only about 50%.